1. Field of the Invention
The present invention relates to a reference signal generation apparatus and a reference signal generation system.
2. Description of Related Art
Optical encoders are widely used to detect a position of a measurement device and the like. In an incremental encoder, a scale for detecting a relative position and a scale for detecting a reference position are provided. The incremental encoder reads the scale for detecting a reference position so as to convert detected relative position information into absolute position information. Therefore, the incremental encoder is required to detect a reference position with high accuracy.
To detect a reference position, there is a known method in which a plurality of light-receiving elements are arranged separately from each other in a reading direction of a reference point detection pattern, and a difference signal between the light-receiving elements is generated. By detecting a point where the difference signal crosses over a zero level (an intersection point), it is possible to identify a position of the reference point detection pattern in the reading direction (e.g., Japanese Unexamined Patent Application Publication No. 10-332431).
As another example of detecting an intersection point, there is a proposed method of correcting a shift in the intersection point pattern using a result of reading a pattern finer than the reference point detection pattern (Japanese Patent Translation Publication No. 2009-515182).
Further, there is another proposed method in which a plurality of light-receiving elements having a small width in the reading direction are arranged in the reading direction, and a logical AND of a difference signal between two adjacent light-receiving elements is obtained, thereby generating a reference signal with a small width (Japanese Unexamined Patent Application Publication No. 2006-329652).
Furthermore, there is another proposed method of providing a plurality of reference point detection patterns and generating a reference point by a difference in timings of reading the respective patterns (Japanese Unexamined Patent Application Publication No. 2000-304574).